Why does Cox bill not decrease when I pay extra?

I have looked over my billing to the past 7 months and have found that even though I pay a few dollars more my bill stays the same. I will be paying $31 extra next month just to see how Cox handles the process. I also have noticed a difference in tax, fees, surcharges from  (3mo) $15.82, (3mo)$14.82, and one mo $15.05. Of the 7mos only 2 had 31-day billing cycle that rest was 30 day.